This invention relates to lubricants for use in the drill pipe tool joints and collar connections. More particularly, it relates to thread lubricants for rotary shouldered connections used in the drill string for telemetering electrical signals.
Before torquing up a rotary shouldered connection, a thread lubricant is typically applied to the pin and the box. This lubricant serves several purposes: It lubricates the threads and the shoulders of the pin and box being screwed together to prevent galling; it creates a desirable amount of preload and friction between the pin and the box under the applied makeup torque to keep them locked together during operation; and it prevents leakage through the connection by sealing off any geometrical imperfections (due to machining or handling damage) between the mating shoulders of the pin and box. Any galling of the mating threads or shoulders is detrimental not only to the mechanical functioning of the connection, but it can also create the deterioration of the electrical link due to contamination by the loose metal slivers created by the galling process. The ability of the lubricant to seal is also important from the standpoint of electrical integrity of the communication link. A leak through the joint, once started, will cause shortage and degradation of the electrical communication, in addition to causing an eventual "washout" due to erosion.
It has been known for over forty years that transmitting an electrical signal along a drill string is desirable. Various proposals have been put forth, but the connection at the pipe joint typically has been a source of breakdown in the communication link. The electrical connectors were part of the mating section of the pipe and thus during assembly and disassembly, were coated with various fluids including pipe thread lubricants. This coating would usually interfere with the transmission of the electrical signal.
The metal-to-metal contact area was relatively small and the effective contact resistance tends to vary widely due to the presence of differing amounts of relatively nonconductive fluids. One type of connector which solved this problem is disclosed in U.S. Pat. No. 3,696,332 which is assigned to Shell Oil Company. This improved connector contains a generally ring-shaped and substantially full-circle contact-making conductive metal portion that is located in an annular groove within a pipe joint element; is electrically insulated from the groove walls; and is electrically connected to a segment of the insulated electrical conductor. At least one member of each pair of mating contact-making rings is mounted adjacent to a resilient biasing means that is arranged to urge the contact-making ring toward a position from which the fluids between the rings are displaced, with a wiping action by the mating ring, as the elements of the tool joint are screwed together.
With this type of connector, a thread lubricant which is nonconductive and which provides performance characteristics similar to other metal containing thread lubricants conforming to API standards is necessary for satisfactory operation. Specifically, this thread lubricant must (1) have anti-galling characteristics to prevent seizure or damage to the tool joints, (2) have adequate lubricating properties to provide a smooth makeup, (3) provide a sealing function by bridging the geometrical imperfections (e.g.: gaps, tool marks, dents, etc.) between the sealing shoulders, and (4) have approximately the same coefficient of friction as the other thread lubricants that meet the API specifications for the drill pipe thread compounds. This coefficient of friction requirement is especially important to assure that tensile stresses created in the pin member under the API recommended makeup torque are within a safe range to avoid permanent stretch. The coefficient of friction of the thread compounds conforming to the API Specification 7 is approximately 0.08. Too low a coefficient of friction will create higher tensile stresses in the pin under the recommended API torque. Alternatively, if the reduced makeup torque is used for a low coefficient of friction thread compound to achieve the same average pin stress as used in the API specification (approximately one-half of the tensile yield strength), the torsional resistance of the tool joint to further makeup downhole is reduced. This can result in a significant damage to the drillstring (i.e.: stretched pins) as well as joints difficult to break out using normal procedures.
A pertinent prior pipe lubricant is disclosed in U.S. Pat. No. 3,526,593 which contains a hydrous calcium complex grease and between 33 and 80 percent Teflon (a trademark of DuPont for polytetrafluoroethylene). The average particle size of the Teflon is between 0.005 to 0.05 inches. Based on our tests, this type of grease would be expected to have a coefficient of friction of approximately 0.05. This low coefficient of friction will result in 60 to 70 percent higher pin stresses than with the use of API conforming thread lubricants for tool joints. It can also result in stretched pins due to further makeup downhole as described above. This thread lubricant described is best suited for sealing threaded and coupled pipe joints used in the casing and tubing, not for the drillstring rotary shouldered connections.
In another pertinent prior lubricant, U.S. Pat. No. 2,961,401, a lubricant is disclosed containing a lubricating oil grease and about 10 to 50 percent finely divided hard plant seed having a compressive strength of at least about 5,000 psi. This thread lubricant is aimed at improving leak resistance along the leakage path between the threads of a coupled joint. In rotary shouldered connections, sealing is achieved by compression of the box and pin shoulders where this lubricant offers no particular advantage.
The object of this invention is to provide a nonconductive pipe lubricant having a coefficient of friction of about 0.08, galling resistance up to 140 percent of the API recommended makeup torque and sealability up to 5,000 psi of differential pressure.
Another object of this invention is the performance of the pipe lubricant at elevated temperatures for extended periods of time.